Much work has been done over the years to convert heavy hydrocarbonaceous materials to more valuable lighter boiling products by various thermal processes including visbreaking, delayed coking and fluid coking.
In fluid coking, a heavy oil chargestock, such as a vacuum residuum, is fed to a coking zone containing a fluidized bed of hot solid particles, usually coke particles, sometimes referred to as seed coke. The heavy oil undergoes thermal cracking at the high temperatures in the coking zone resulting in conversion products which include a cracked vapor fraction and coke. The coke is deposited on the surface of the seed coke particles and a portion of the coked-seed particles is sent from the coking zone to a heating zone which is maintained at a temperature higher than that of the coking zone. Some of the coke is burned off in the heating zone and hot seed particles from the heating zone are returned to the coking zone as regenerated seed particles, serving as the primary heat source for the coking zone. In the variant of the fluid coking process developed by Exxon Research and Engineering known as Flexicoking™, a portion of hot coke from the heating zone is circulated back and forth to a gasification zone which is maintained at a temperature greater than that of the heating zone. In the gasifier, substantially all of the remaining coke on the coked seed particles is burned, or gasified, in the presence of oxygen (air) and steam to generate low heating value fuel gas which can be partly passed to the burner/heater to increase temperature in that zone and/or used as refinery fuel. Fluid coking processes, with or without an integrated gasification zone, are described, for instance in U.S. Pat. Nos. 3,726,791; 4,203,759; 4,213,848; and 4,269,696.
Modifications have been made over the years in an attempt to achieve higher liquid yields. For example, U.S. Pat. No. 4,378,288 discloses a method for increasing coker distillate yield in a coking process by adding small amounts of a free radical inhibitor. Notwithstanding these improvements, however, there remains a need for process and equipment modifications which can increase liquid yields and in fluid coking, a reduction of the temperature in the coking zone is the most effective solution. While there are economic incentives to increase the feed capacity, reducing the temperature of the coking zone and increasing unit capacity will tend to increase the amounts of liquid hydrocarbon passing from the coking zone to the stripping zone with consequent increase in the fouling in the stripping zone. Various techniques for alleviating the fouling problem have been proposed: US 2011/114468, for example, describes the use of perforated sheds in the stripping zone while U.S. 2011/0206563 describes the use of downwardly slopping frusto-conical baffles in the coking zone to the same end. Nevertheless, the objective of increasing the yield of the desired liquid products remains with the desirability of reducing reactor temperature even in the face of the fouling problem which is created by reductions in reactor temperature.
By increasing the temperature of the stripping zone the liquid yield may be increased by enabling the temperature of the coking zone to be reduced. U.S. Pat. No. 5,176,819 describes a process to run the stripping zone at a higher temperature than the coking zone by feeding a portion of the heated solids from the burner/heater (and gasifier if applicable) to the stripping zone. Significant liquid yield increases of 1% are reported while the increased temperature of the stripping zone also tends to reduce the amount of hydrocarbon carryunder out of the stripping zone. We have now found that the flow in the fluid bed coking unit, especially in the coking zone, is dominated by large scale recirculation patterns that are much faster (˜50×) than the external circulation rate between the coking zone and the burner/heater/gasifier. This suggests that the hot solids from burner/heater/gasifier fed to the stripping zone in the manner described in U.S. Pat. No. 5,876,819 could be recirculating in both the stripping zone and the coking zone: the hot coke fed to the top of the stripper becomes distributed in both the coking zone and the stripping zone and the mass fraction of the hot coke in the coking zone and in the stripping zone is similar. This indicates that the coking zone and the stripping zone are not effectively decoupled and that the coking zone is not being operated at the desired relatively lower temperature with a consequent loss in liquid yield and, conversely, that the stripping zone is not being operated at the higher temperature appropriate to reduce fouling.